One might ask: but nuclear weapons are devastating. Why would you need big space rocks ? Nuclear weapons have mechanisms which can be destroyed prior to the time that they get within a few kilometers of their target. Space rocks are destructive because of kinetic energy. Even if you blast the rock to pieces, all of the pieces still retain the energy. If you can blast the electronics of the nuclear device it could be stopped from exploding. Plus as seen in the calculator space rocks can already by very big and fast and big enough to be trillions of times as destructive. A big space rock could create a thousand mile crater and melt the top several miles of the planet.

So if you have advanced space travel, space construction capabilities and metamaterials: you can go to big space rocks, set up camp, establish the means of steering them, use metamaterials and make an invisible shell. Maneuver them into suitable station keeping -ready to use orbits. Maintain communications and protocols with the side that you work for. Then your side has invisible doomsday devices. They are usable for a very messy first strike. Very good for second (retribution) strikes for assured destruction of enemies. The second strike would be good for rational powers to use to deter rational opponents. All rational people would just need to agree to get rid of/completely neutralize those who are dangerously irrational and/or nihilistic.